After two decades of fighting the "War on Cancer," physicians and public health specialists now realize that more effort must be devoted to prevention. Lay persons realize this, also. Americans spent $82 million per year on just one compound thought to prevent cancer (beta-carotene). The diet is a bountiful source of potentially chemopreventive compounds. Epidemiological studies have shown that populations consuming large amounts of soybeans have less breast, colon, and prostate cancer than do Americans. Animal studies have confirmed the cancer protective effect of soybeans. Soybeans contain numerous anticancer agents, including the isoflavones, daidzin and genistin. This study examines underlying mechanisms responsible for breast cancer reduction by phytoestrogens. The hypothesis to be tested is that consumption of soymilk containing phytoestrogens reduces circulating ovarian steroids and gonadotropins. Such changes may be oncoprotective. Premenopausal women with regular menstrual cycle (20-30 years old) and not on birth control pills will be placed on soybean containing diet for 1 month. Their hormone levels will be measured for 3 months before soya feeding, during 1 month of soya feeding and for at least 5 months after withdrawal from soya feeding. The subjects will consume 0 - 100 mg of daidzein and 0 - 100 mg of genistein (the two phytoestrogens in soybeans) daily on a Clinical Research Center. This study will determine the quantity of phytoestrogen consumption needed to reduce ovarian steroids and gonadotropins in premenopausal women. In separate studies, postmenopausal women will ingest soymilk for 4 months and provide blood, urine, vaginal smears, and endometrial biopsies for the analyses of steroids, isoflavone metabolites, cytology, and histology, respectively. These women will also be examined for endometrial thickness by vaginal probe ultrasound. These studies will determine estrogenicity of long term phytoestrogen consumption. Established immunoassays will be used to analyze hormone levels. This study will have significant implications for the health of American women and for the economy of the U.S., which is a major soybean producer.